The present invention relates to a dry end for a paper making machine and particularly to cutting the web for threading its leading end into a dryer group. Dry ends which are formed of so called single wire dryer groups are known from:
1. "Wochenblatt fur Papierfabrikation," No. 22, 1988, pg. 6; PA1 2. International Application WO 90/01580; PA1 3. International Application WO 90/02225; PA1 4. German Utility Model G 90 01 209.
These publications disclose that dryer cylinders can be arranged, at least in the predominant part of the dry end, in either at least approximately horizontal or at least approximately vertical rows. Such a dry end is divided into several dryer groups. Each dryer group comprises a plurality of heatable dryer cylinders, a plurality of deflecting rolls, with one deflecting roll between each neighboring pair of dryer cylinders, and a support belt which presses the web to be dried against the dryer cylinders.
Publication 4 above shows successive dryer groups of a dry end, wherein one side of the web initially comes into contact with the outer surfaces of the dryer cylinders in the first group and the other side of the web comes into contact with the outer surfaces in the dryer cylinders of the second group. In contrast, in the dry end, in application 3, the same side of the web comes into contact with the outer surfaces of the individual cylinders of both or of all dryer groups. Both designs were considered for the purpose of the present invention.
Applications 2 and 3 disclose a web tip cutter which is installed at the outlet end of the dry end. As long as the web travels in the broke pulper, a narrow edge strip can be cut from the web by the web cutter. That edge strip then serves for threading the web into the following treatment stations, for instance, a calendar and reeler, a glue press, or the like. For such a tip cutter, a free length of web is formed by means of a special arrangement of rolls because upon the cutting of the tip, the web must be free of the support belt which otherwise travels along together with the web.
In Application 2, a free length of the web is formed at the place of separation between the next to the last and the last dryer groups and therefore at a place where the web is still not completely dry. As a result, there is the danger that the web may tear upon the cutting of its tip. On the other hand, if necessary, use may be made of the possibility of controlling the tension of the web upon its being cut by the tip, by changing the differential of the speeds between the two dryer groups.
In Application 3, a free length of the web is formed within the last dryer group, preferably between the last two dryer cylinders, and therefore at a place where the web is completely dry. Thus, the web should only rarely tear upon its being cut by the tip. There is a deflecting roll with which the support belt comes into direct contact and between the last two dryer cylinders. In addition, a web guide roll is arranged below this deflecting roll. The web travels free of the support belt, from the deflecting roll to the web guide roll and from the latter back to the deflecting roll so that two free lengths of web are present. The tip cutter is arranged on one of these two free lengths of web. This arrangement has the danger that upon tearing of the web or upon some other unstable operating condition, the web may wind up on the web guide roll. Furthermore, it has the disadvantage that there are two free lengths of web although only a single free length of web is required for the tip cutter. The danger of tearing, which is always present on every free length of web, is therefore unnecessarily increased.
Application 3 discloses a dry end in which a transfer or separation between two successive dryer groups is neither shown nor plays a part. It concerns forming open lengths of the paper web at which a cutter can be inserted. In this case, two successive dryer cylinders are wrapped by the paper web and by the felt support belt web jointly such that the paper web is contained in sandwich like fashion between the outer surface of the corresponding dryer cylinder and the support felt which travels on its outer side. Between the two dryer cylinders, two guide rolls are arranged. The guide roll that is closer to the dryer cylinder is wrapped around solely by the felt support belt, while the other guide roll which is further from the dryer cylinders is wrapped around solely by the paper web. Thus, free lengths of the paper web are formed between the two guide rolls. The tip cutter can act on these free lengths. The free lengths are thus continuously present, with all the risks which are inherent in high machine speeds, particularly the risk of fluttering and tearing.
German Patent Application P 40 12 246, which does not constitute a prior art publication, discloses a dry end which is comprised of single line dryer groups. Two guide rolls are again provided within a dryer group between two neighboring dryer cylinders. Between these two guide rolls there is a central guide roll. The central guide roll is wrapped around solely by the support belt so that a free length of the paper web is produced between the lower and the upper guide rolls (see FIG. 3 thereof). A tip cutter can be placed on this free length of the paper web.
This embodiment is advantageous, as compared with that shown in Application 3 above, since only a single free paper web length is present instead of two. However, this free length of paper web is also present continuously, and therefore also during the operation of the paper making machine at high speed.